The present invention relates to methods and apparatus for detecting transfer leaks in solid polymer electrolyte fuel cells and locating such cells in fuel cell stacks.
Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
Solid polymer fuel cells employ a solid polymer electrolyte, or ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The membrane is typically proton conductive and acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically interposed between two plates to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or circulating coolant. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure gaseous hydrogen or a reformate stream comprising hydrogen, or a liquid such as, for example, aqueous methanol. The fuel fluid stream may also contain other fluid components such as, for example, nitrogen, carbon dioxide, carbon monoxide, methane, and water. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen supplied as, for example, substantially pure gaseous oxygen or a dilute oxygen stream, such as, for example, air, which may also contain other components such as nitrogen, argon, water vapor, carbon monoxide, and carbon dioxide. Various sealing mechanisms are used to fluidly isolate the fuel and oxidant streams from one another in the fuel cell.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also needed to control the temperature within a fuel cell assembly to prevent overheating. Conventional fuels cells employ a liquid, such as, for example, water to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Thus, conventional fuel cells typically employ three fluid streams, namely fuel, oxidant, and coolant streams, which are fluidly isolated from one another. See, for example, U.S. Pat. No. 5,284,718 and FIGS. 1, 2A and 2B of U.S. Pat. No. 5,230,966, which are incorporated herein by reference in their entirety, for examples of typical fuel cell assemblies configured to fluidly isolate the aforementioned three fluid streams. Fluid isolation is important for several reasons. For example, one reason for fluidly isolating the fuel and oxidant streams in a hydrogen-oxygen fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, the membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen. However, since the membrane also functions as an electrolyte, the membrane is generally permeable to protons and water. (Water is generally required for proton transport in membrane electrolytes.)
The coolant fluid is preferably isolated from the reactant fluids to prevent dilution and contamination of the reactant streams. Furthermore, water may cause flooding in the reactant fluid passages which prevents the reactants from reaching the electrochemically active membrane-electrode interface. It is also undesirable for the reactant streams to leak into the coolant stream because this reduces operating efficiency as the leaked reactants are not used to generate electrical power. Likewise, leakage of any of the fluids to the surrounding atmosphere is generally undesirable.
There are several conventional methods of detecting leaks. For example, in a hydrogen-oxygen fuel cell, the oxidant exhaust stream can be monitored to detect the presence of hydrogen. When hydrogen is detected in the oxidant exhaust stream, this may indicate a leak. A problem with this method is that hydrogen may be present in the oxidant exhaust stream for reasons other than a leak. For example, if there is a shortage of oxygen at the cathode, protons arriving at the cathode from the anode may recombine with electrons to form hydrogen. There are many possible causes for such an oxygen shortage. For example, an oxygen shortage may result from a sudden increase in power output demand, a malfunctioning compressor, a blockage in fluid flow field channels caused by an accumulation of product water, or a clogged air filter.
An additional problem with using a constituent such as hydrogen, other reactants, or reaction products, as an indicator of a leak is that these constituents may be reactive within the fuel cell. These constituents may be particularly reactive in the presence of the electrocatalyst at the interfaces between the electrolyte and the anode and cathode. Consequently, these substances may react partially or completely prior to being exposed to a detector located in the fluid exhaust manifold. Thus, the concentration of any detected substances may not accurately reflect to the amount of the constituent substance that is leaking and may prevent or delay the detection of a leak.
When the fuel stream comprises carbon dioxide, a method of detecting leaks between the fuel and oxidant fluid streams involves detecting greater than a threshold level of carbon dioxide in the oxidant exhaust stream. A disadvantage of this method is that an oxidant supply stream, such as air, may already comprise carbon dioxide in varying concentrations. This may be especially true in vehicular applications where the oxidant intake may receive air comprising the exhaust streams of other vehicles. Therefore, a disadvantage of this method is that for reliable operation, it is necessary to measure the carbon dioxide concentration in the oxidant intake stream, as a reference, in addition to measuring the carbon dioxide concentration in the oxidant exhaust stream.
Another method of detecting leaks between the fuel and oxidant fluid streams is to measure the oxygen concentration in the fuel exhaust stream. Like the aforementioned methods, a problem with this method is that there are other potential sources of oxygen at the anode. For example, sometimes oxygen is introduced into fuel reformate supply streams to counter the effects of catalyst poisoning. Another source of oxygen at the anode is water that may be converted to oxygen, electrons, and protons at the anode when there is a shortage of fuel (that is, fuel starvation). Therefore, a disadvantage of these oxygen detection methods is that other parameters must be analyzed to determine when the oxygen measured within the fuel exhaust stream is the result of fuel starvation, a leak, or residual oxygen that was added to the fuel supply stream.
Fuel cells are also typically checked for leaks prior to operating the fuel cell to produce power, for example, after assembly or during routine maintenance. Another method of checking for leaks is to introduce a gas into the inlet of one of the fluid passages while the outlet is sealed. The other fuel cell fluid passage inlets are sealed and the outlets are typically fluidly connected to a bubble tube. The volume of any gas that bubbles through the bubble tube is measured to determine if there is any leakage. A problem with this test is that it is difficult to administer with consistent results and the test is a time consuming one. Also, particularly with respect to the reactant fluid passages, the pressurization of only one reactant fluid passage may result in damage to the thin membrane electrolyte layer and/or other fuel cell components.
Further, in the foregoing methods it may not be possible to easily and reliably determine which fuel cell(s) in the stack are leaking, if a leak is detected. Thus, it is often necessary to disassemble the stack and re-test it in sections until each leaking cell is found. Such a time-intensive, iterative process is less than desirable.
Accordingly, there is a need for a simple and reliable method of detecting a leak in a fuel cell stack. That is, a method that provides a rapid indication of a leak and that can identify the fuel cell or cells that are leaking.
A method of detecting transfer leaks within a fuel cell stack is provided. In one embodiment of the present method, the stack comprises: a plurality of fuel cell assemblies each comprising a membrane electrode assembly comprising an anode, a cathode, and an ion exchange membrane interposed therebetween; at least one fuel manifold fluidly connected to the fuel stream passages of the fuel cell assemblies; and, at least one oxidant manifold fluidly connected to the oxidant stream passages of the fuel cell assemblies. The embodiment of the present method comprises:
(a) supplying inert gas to the oxidant manifold(s) at a first pressure;
(b) supplying fuel to the fuel manifold(s) at a second pressure greater than or equal to the first pressure;
(c) applying a potential difference across at least one of the fuel cell assemblies such that the cathode(s) thereof is/are more positive than the anode(s) thereof; and
(d) measuring the transfer current across the fuel cell assembly or assemblies.
In another embodiment of the present method, the fuel cell stack further comprises coolant stream passages in thermal communication with at least a portion of the fuel cell assemblies, and at least one coolant manifold fluidly connected to the coolant stream passages. This embodiment of the present method comprises:
(a) supplying fuel to the fuel manifold(s) at a first pressure;
(b) supplying inert gas to the oxidant manifold(s) at a second pressure;
(c) supplying fuel to the coolant manifold(s) at a third pressure;
(d) applying a potential difference across at least one of the fuel cell assemblies such that the cathode(s) thereof is/are more positive than the anode(s) thereof; and
(e) measuring the transfer current across the fuel cell assembly or assemblies.
In operation, the third pressure is greater than at least one of the first and second pressures. The first and second pressures may be substantially the same, or the first pressure may be greater than the second pressure.
Yet another embodiment of the present method comprises:
(a) supplying inert gas to the fuel manifold(s) at a first pressure;
(b) supplying fuel to the oxidant manifold(s) at a second pressure;
(c) supplying fuel to the coolant manifold(s) at a third pressure;
(d) applying a potential difference across at least one of the fuel cell assemblies such that the cathode(s) thereof is/are more positive than the anode(s) thereof; and
(e) measuring the transfer current across the fuel cell assembly or assemblies.
In operation, the third pressure is greater than at least one of the first and second pressures. The first and second pressures may be substantially the same, or the first pressure may be greater than the second pressure.
In the foregoing embodiments, the pressure differential between the first and second pressures may be between 0 kPa and about 70 kPa, preferably between about 6.5 kPa and about 35 kPa. The inert gas may be selected from the group consisting of nitrogen, argon, helium, and carbon dioxide. The potential difference may have a magnitude of about 0.2 V to about 0.9 V, preferably of about 0.5 V.
The present method may further comprise comparing the measured transfer current with a reference transfer current.
An apparatus for detecting transfer leaks within a fuel cell stack is also provided. In one embodiment, the stack comprises a plurality of fuel cell assemblies, at least one fuel manifold fluidly connected to the fuel cell assemblies, and at least one oxidant manifold fluidly connected to the fuel cell assemblies. The embodiment of the present apparatus comprises:
(a) a fuel source fluidly connectable to the fuel manifold(s) for supplying fuel thereto at a first pressure;
(b) an inert gas source fluidly connectable to the oxidant manifold(s) for supplying inert gas thereto at a second pressure;
(c) a DC power supply connectable to at least one of the fuel cell assemblies; and
(d) at least one device for measuring the transfer current across the fuel cell assembly or assemblies.
The apparatus may further comprise a transfer current display for receiving the output signal from the device(s).
In another embodiment of the present apparatus, the fuel cell stack further comprises coolant stream passages in thermal communication with at least a portion of the fuel cell assemblies, and at least one coolant manifold fluidly connected to the coolant stream passages. This embodiment of the present apparatus comprises:
(a) a first fuel source fluidly connectable to one of the fuel and oxidant manifolds for supplying fuel thereto at a first pressure;
(b) an inert gas source fluidly connectable to the other of the fuel and oxidant manifolds for supplying inert gas thereto at a second pressure;
(c) a second fuel source fluidly connectable to the coolant manifold(s) for supplying fuel thereto at a third pressure;
(d) a DC power supply connectable to at least one of the fuel cell assemblies; and
(e) at least one device for measuring the transfer current across the fuel cell assembly or assemblies.
The first fuel source and second fuel source may be the same or different. Similarly, the first and second pressures may be substantially the same or different. The apparatus may further comprise a transfer current display for receiving the output signal from the device(s).
In yet another embodiment of the present apparatus, the stack comprises a plurality of fuel cell assemblies, a fuel source fluidly connected to the stack and adapted to supply it with fuel at a first selected pressure, and an oxidant source fluidly connected to the stack and adapted to supply it with oxidant at a second selected pressure. This embodiment of the present apparatus comprises:
(a) a DC power supply connectable to at least one of the fuel cell assemblies;
(b) at least one device for measuring the potential difference across the fuel cell assembly or assemblies, and for generating an output signal representative of the measured potential difference; and
(c) a display for receiving the output signal from the device(s).